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Recognition Elements

This page is reproduced with the permission of the author Doug Dudycha
from the University of Waterloo. The original is here.

Airphoto interpretation is the process of viewing airphotos, identifying
geographic features represented in airphotos based on image characteristics,
and relating image characteristics to known ground conditions in order
to obtain information about things you can't see in the airphoto. For example,
an experienced interpreter can distinguish between high and low income
areas on an airphoto based on looking at lot and building size and on associations
between features such as presence of swimming pools, lots backing onto
a golf course, etc.

Several image characteristics may be used to identify features and interptet
ground conditions. These include pattern, shape, tone, texture, shadow,
associated features, and size.

Patterns can help to identify natural, agricultural and urban features.
Natural patterns often reflect surficial bedrock geology or dominant geomorphic
processes. For example, evidence of glaciation may be found in scraping
of the topsoil from bedrock or in depositional features such as drumlins
or moraines. Patterns can also be used to differentiate agricultural features.
For example, orchards and vineyards show distinctive spatial patterns.
Fields subjected to circular irrigation are also clearly evident on airphotos,
as are settlement patterns derived from splitting large blocks of land
into smaller farms. In urban landscapes, patterns can help distinguish
between residential, commercial and industrial areas and may even allow
you to differentiate residential areas based on their age.

Shape is particularly important in interpretation of urban images. Shapes
can help distinguish between different building types. Roof shape often
provides a clear indication of the type of structure which may help in
identifying its function.

Tone can be a useful image characteristic but can also be problematic.
Tone tends to vary too much across the image, in part because tone is affected
by shadows of objects in the image. Airphotos are usually taken in late
morning, so the sun angle is typically from the southeast. Because of radial
displacement of objects from the nadir, we see the sides of objects as
well as the tops of objects in the airphoto. However, in the southeast
quadrant of the image, we see the shadowed northwest sides of objects,
producing a darker tone, while in the northwest quadrant, we see the sunlit
sides, producing a lighter tone. This variation in tone can make interpretation
more difficult. Tone may be used to delineate drainage networks since wetter
soils will have a darker tone, but darker tones could also be caused by
the presence of organic soils.

Texture is particularly important in interpreting vegetation types.
Not only is ti possible to distinguish between broad forest classes such
as deciduous vs coniferous forest, but an experienced interpreter can also
distinguish varieties of trees, e.g. red maple vs sugar maple or cherry
trees vs peach trees, based on the texture of the image. Detailed information
about forest stands can be interpreted from airphotos, as has been done
in producing Ontario's Forest Resource Inventory (FRI) maps. These maps
are derived primarily through image interpretation with some field checking
and describe the age and species composition of individual forest stands.

Shadows can reveal the types of structure or allow differentiation of
different types of trees. Shadows are more pronounced on low sun angle
photographs, making identification of feature types easier. However, shadows
may hide detail in the image and affect the tone of the image which may
make interpretation more difficult.

Many types of features can be easily identified by examining associated
features. For example, a public school and a high school may be similar
flat roofed building structures but it may be possible to identify the
high school by its association with an adjacent football field and track.
Similarly, a light industrial building and a shopping plaza may be difficult
to distinguish based on the building structure type but the shopping plaza
will be associated with a larger parking area than the industrial building.

The size of objects can also aid in interpretation. A cemetary and a
campground may appear similar on an airphoto image since both show a regular
spatial pattern of paths/roads and rectangular objects. In this case, the
size of the objects can be used to aid in interpretation, although it may
be necessary to determine the scale of the image to arrive at the correct
interpretation.

3-D Airphoto Interpretation

Because of the overlap between successive airphotos along a flight, it
is possible to view airphotos stereoscopically, i.e. in three dimensions.
However, stereoscopic viewing is limited to the area of overlap between
the images.

Stereoscopic viewing is based on binocular vision. Each of our eyes
sees a scene from a slightly different perspective. Our brains reconstruct
the two images recorded by our eyes into a three dimensional view of the
scene. The same thing is possible with airphotos (or other images) provided
that when we view the airphotos, each eye is focused on a single image.

There are several methods that can be used to ensure that each eye sees
only one of a pair of images. Early 3-D movies relied on the use of analgyphs.
To see the movie in 3-D, the audience was required to wear glasses with
red and green lens. The coloured lenses filter out different colours, so
each eye sees a different image which the brain reconstructs into a perspective
view. Polarized light and projectors operate in a similar way. By changing
the direction of polarization, each eye views a different image.

In airphoto interpretation, stereoscopic viewing is usually assisted
by the use of pocket of mirror stereoscopes. Both operate on the same principle.
Mirror stereoscopes have the advantage of being able to view larger images
than is possible with a pocket stereoscope which is limited by the approximately
5 cm distance between our eyes. We look at a pair of overlapping airphotos
through lenses that force each eye to see only one of the pair of photos.
Once again, our brain reconstructs the three dimensional view from the
pair of images.

Pocket Stereoscope

Mirror Stereoscope

Depth perception is a function of the parallax angle, which is the angle
between the eyes and an object in a pair of stereo images. The parallas
angle decreases with distance from the object. Because of radial displacement
of objects in the image, the top of an object appears to be at a different
depth than the bottom of an object.

Parallax Angle

In setting up airphotos for stereoscopic viewing, care must be taken
to avoid psuedoscopic vision. Psuedoscopic vision can occur in two ways:
if the order of the airphotos is reversed or if the shadows in the image
point away from the observer. Both of these conditions will cause the 3-D
image to appear to be inverted.

Psuedoscopic Vision

A final problem with three dimensional viewing of airphotos is vertical
exaggeration. Objects in the image appear to be taller than in reality
and slopes appear to be steeper. This exaggeration can sometimes aid in
interpretation but is somewhat disorienting to inexperience viewers. Vertical
exaggeration occurs because of the difference in geometry when taking the
airphotos and when viewing the airphotos. Vertical exaggeration varies
with camera focal lenght and % overlap between successive images. Vertical
exaggeration can be calculated as:

VE = ( B / H ) / ( b / h)

where: B is the air base; H is the height of the aircraft above the ground;
b is the eye base (approximately 6 cm) and h is the distance from the eye
at which the stereo model is perceived (approximately 45 cm)

Vertical Exaggeration

Multi- Concept

Many applications of airphoto interpretation require interpretation of
mulitple images. This can include use of multi-scale images, multi-temporal
images and multi-spectral images.

Multi-scale images require a series of images at different scales, taken
at the same time. Although simultaneous acquisition is difficult, if not
impossible, it is often possible to acquire images from different sources
that were taken at approximately the same time, i.e. within a few days
of one another. Multi-scale images could include satellite-based Landsat
MSS, Landsat Thematic Mapper or SPOT images, airborne MEIS or CASI images,
and airphotos taken from different flying heights or using different camera
lenses. In general, in interpreting multi-scale images, we use the larger
scale images to interpret smaller scale imagery. Alternatively, smaller
scale imagery may be used for reconnaisance purposes and larger scale imagery
for more detailed analysis within selected sub-areas of the smaller scale
image.

Multi-temporal images are used to analyze landscape change over time.
Examples could include examining changes in river systems or sand dunes,
monitoring crops over a growing season to forecast crop yields, or monitoring
urban growth. In these types of application, we are using images of the
same area acquired at different points in time.

Multi-spectral imagery is often used to aid in interpretation of specific
types of features. For example, colour IR film clearly distinguishes water
from land and is useful at distinguishing between different vegetation
types which may be hard to interpret from normal black and white or colour
airphotos. In this case, we can select spectral bands that are best suited
to identifying the types of features we are interested in. We can also
combine spectral bands to create a new index that may be more revealing
than the individual bands alone.

Applications

There are numerous potential applications of airphoto interpretation. Airphoto
interpretation has been widely used as the basis for land use classification
and mapping, and for mapping changes in land use over time. In developing
countries that often do not have reliable population databases, airphoto
interpretation can be used to estimate housing density. By calculating
the housing density for representative sample areas with an airphoto image,
reliable estimates of housing density can be obtained for other similar
areas in the image. If information is available on average household size,
then this method can be extended to produce estimates of population density.

Airphotos have often been used in transportation studies and can be
used to identify vehicle types, estimate traffic flows, identify parking
problems on city streets, estimate parking lot usage, and even to measure
the speed of vehicles on a highway.

Airphotos are regularly used in the aftermath of natural disasters such
as earthquakes, volcanic erupttions or floods, to guide relief efforts.
Insurance companies also make use of airphotos to assess damage and verify
insurance claims.

Some municipalities use airphotos to identify building code violations
and enforce compliance with permitting procedures. Most municipalities
required building permits for any construction project larger than a small
backyard shed. New construction can be identified on an airphoto and permit
records can be checked to verify that a building permit was issued for
the project. This type of application requires large scale imagery such
as 1:5,000.

Airphoto interpretation has often been used to aid in locating businesses
or public facilities such as schools, fire stations or libraries. By specifying
a set of criteria that represent desireable locations for the business
or public facility, airphoto interpretation can be used to identify sites
that satisfy project requirements. In a similar manner, airphoto interpretation
can be used to do avoidance screening. The objective here is to identify
areas where development cannot occur. This could include areas of steep
slopes, organic soils, buffer zones around marshes, rivers, shorelines
or top of steep slopes, ecologically sensitive areas, conflicting land
uses, class 1 and 2 agricultural land, or gravel deposits. An experienced
interpreter can quickly identify these constraint areas on a airphoto,
often by tracing their outlines on an acetate overlay. While this type
of analysis is increasingly being done using geographic information systems,
manual airphoto interpretation can be much faster than the time required
to develop the GIS database.